Currently, hydrogen gas for fuel cell applications is supplied from devices that store hydrogen either as compressed hydrogen gas, as cryogenic liquid hydrogen, as hydrogen atoms at low density in metallically bonded solid transition metal hydrides, as hydrogen atoms at high density in ionically bonded solid light metal hydrides, as hydrogen atoms at high density in polar covalently bonded solid complex hydrides, or as hydrogen molecules adsorbed on high surface area supports. Each of these methods has shortcomings.
Storage of hydrogen as a compressed gas requires high pressures, approaching 700 bar, in order to achieve acceptable storage densities. These pressures require significant energy for compression while also imposing engineering and safety challenges.
Liquefaction of hydrogen consumes 30% of the energy content of the hydrogen. Liquid hydrogen, at a temperature of 23 Kelvin, is also difficult to maintain over extended times without significant loss due to boil-off.
Transition metals store hydrogen as chemically bonded hydrogen atoms and, therefore, input of energy is needed to release the hydrogen. This energy is input in the form of heat to achieve elevated temperatures. The required temperatures are moderate because of the relatively weak metallic bonding that exists in transition metal hydrides. However transition metal atoms have atomic weights of greater than approximately 50 atomic mass units and store at most approximately two hydrogen atoms per transition metal atom. The hydrogen storage capacity of a storage medium may be quantified as the mass fraction of hydrogen when the medium is saturated with hydrogen. In the case of transition metal hydrides, for example, the storage capacity, which may also be referred to as the gravimetric storage density, is less than 4 percent by weight (wt %) hydrogen, which is too low for many applications.
Light metal atom hydrides can have high hydrogen storage capacities, up to approximately 12 wt % hydrogen. However, the ionic chemical bonds between the metal and the hydrogen in these hydrides are very strong and, therefore, very high temperatures, such as 280° C. and up to over 900° C., are needed to release the hydrogen. These temperatures are impractical for many applications.
Hydrogen stored in polar covalently bonded light metal complex hydrides can have hydrogen storage capacities up to 18 wt % hydrogen. Like light metal hydrides, these compounds are generally very strongly bound and therefore the high temperatures are required to release the hydrogen.
Hydrogen molecules, adsorbed on high surface area supports, are weakly bound. As a result, at moderately high pressures, for example at approximately 100 bar, cryogenic temperatures (typically 77 Kelvin, which is the temperature of liquid nitrogen) are needed to achieve high hydrogen storage capacities. These high surface area supports include nanoporous polymers, which have been reported to store in excess of 4 wt % hydrogen at approximately 75 bar and 77 K.
At room temperature, which avoids the need for cryogenic cooling, the hydrogen storage capacities of high surface area adsorbents are generally too low for widespread practical applications. For example, one of the best performing activated carbon materials, MSC-30, which has a surface area of 2680 m2/g, may have a hydrogen storage capacity of 1.2 wt % at 340 bar, while a zeolited-templated carbon material with a BET surface area of 3800 m2/g (i.e., a surface area measured using the method of Brunauer, Emmett, and Teller), has a hydrogen storage capacity of 2.2 wt % at 340 bar. For these and many other porous carbon materials the hydrogen storage capacity varies approximately linearly with specific surface area.
Hydrogen may also be stored by dissolution into an appropriate liquid solvent because hydrogen is soluble to some extent in most solvents, but the dissolved hydrogen content, i.e., the hydrogen storage capacity, is much too low for practical hydrogen storage. For example, at room temperature the solubility of hydrogen in hexane gives a hydrogen storage capacity of only 0.18 wt % hydrogen at 100 bar, and only 0.97 wt % at 700 bar.
Recently, as described for example in the '309 Application, the possibility of hydrogen storage in nano-confined liquids has been proposed. When confined within a porous solid, the solubility in a variety of liquids of several gasses including hydrogen has been reported to increase up to 50 times. A disadvantage of this approach is that the liquid solvents typically have appreciable vapor pressures, and, as a result, the discharged hydrogen may be contaminated by the solvent vapor to an extent which is unacceptable in some applications. Proton exchange membrane fuel cells, for example, require high purity hydrogen because the platinum (Pt) catalyst, which dissociates the hydrogen at the fuel cell anode, is very susceptible to poisoning from contaminants such as solvent vapors. Thus, when nano-confined liquids are used to store hydrogen for subsequent use in a proton exchange membrane fuel cell, additional measures are required in order to purify the hydrogen after recovery from the storage material. Moreover, over the course of repeated cycles of hydrogen storage and recovery, there will be a steady loss of the solvent liquid.
Thus, there is a need for a hydrogen storage medium with high hydrogen storage capacity at or near standard temperature and pressure, with low energy cost for storage and recovery, and which provides for the recovery of high purity hydrogen.